Conventionally, an exhaust gas purification apparatus provided for an exhaust gas passage of an internal combustion engine has been developed, in which a selective catalytic reduction NOx catalyst (hereinafter referred to as “SCR catalyst”) is carried on a filter. The filter traps the particulate matter (hereinafter referred to as “PM”) contained in the exhaust gas. The SCR catalyst reduces NOx contained in the exhaust gas by using ammonia (NH3) as a reducing agent. The filter, which carries the SCR catalyst as described above, is hereinafter referred to as “SCRF” in some cases.
When SCRF is adopted as the exhaust gas purification apparatus, it is possible to further decrease the size of the exhaust gas purification apparatus as compared with a case in which the filter and the SCR catalyst are separately provided for the exhaust gas passage. Therefore, it is possible to improve the installation performance of the exhaust gas purification apparatus. Further, when SCRF is adopted, the SCR catalyst can be arranged on the more upstream side in the exhaust gas passage. The more upstream the SCR catalyst is arranged in the exhaust gas passage, the more easily the SCR catalyst is heated by the heat of the exhaust gas. Therefore, it is possible to improve the warming-up performance of the SCR catalyst, and it is possible to improve the NOx purification rate of the SCR catalyst.
Patent Document 1 discloses a system comprising an oxidation catalyst, an injector, SCRF, and a slip oxidation catalyst which are provided in this order from the upstream side along with the flow of the exhaust gas in an exhaust gas passage of a diesel engine. The injector is a device which injects ammonia or an ammonia precursor into the exhaust gas. The slip oxidation catalyst is a catalyst which oxidizes ammonia allowed to pass through SCRF.
Patent Document 2 discloses an exhaust gas purification catalyst comprising an oxidation catalyst, a three-way catalyst, and an HC adsorbing material. Further, Patent Document 2 discloses the following fact. That is, the exhaust gas purification catalyst contains the HC adsorbing material to provide such a concentration distribution that the amount thereof is large on the upstream side and the amount thereof is small on the downstream side along with the flow of the exhaust gas. Further, the exhaust gas purification catalyst contains the oxidation catalyst or the three-way catalyst to provide such a concentration distribution that the amount thereof is small on the upstream side and the amount thereof is large on the downstream side along with the flow of the exhaust gas.